1. Field of the Invention
The present invention relates to a photomask and a method for manufacturing a semiconductor device using the photomask.
2. Description of the Related Art
A field-effect transistor is used as a switching element for controlling a potential of a pixel electrode of a display device typified by a liquid crystal display device and a light-emitting display device. A channel region and a wiring of the field-effect transistor are formed by etching of a semiconductor layer and a conductive layer, respectively, with the use of a resist mask formed by a photolithography step using a photomask.
Owing to high integration of a semiconductor device, light-blocking portions of a photomask are densely arranged. In the photolithography step using the photomask, there is a problem in that the shape of a corner portion of the resist mask is distorted by an optical proximity effect to become different from the shape of the photomask. This problem is particularly significant in formation of a minute pattern with dimensions near the resolution limit in a projection optical system which is used in a photolithography step. For example, in the photomask illustrated in FIG. 6A in which a light-blocking portion 81 is formed over a light-transmitting substrate 41, the intensity of light projected from a light-exposure machine is too high at a corner portion shown by a dashed line 82 which has an angle of greater than 0° and less than 180° (90° here). On the other hand, at a corner portion shown by a dashed line 83 which has an angle of greater than 180° and less than 360° (270° here), the intensity of light projected from the light-exposure machine is insufficient.
Therefore, when a resist is applied to a conductive layer 84 formed over a substrate 43 and light is projected from the light-exposure machine to the resist by using the photomask, a corner portion shown by the dashed line 82 and a corner portion shown by the dashed line 83 of a resist mask 85 do not reflect the shape of the light-blocking portion 81 of the photomask and thus are rounded (see FIG. 6B).
Etching of the conductive layer 84 using the resist mask 85 forms a conductive layer 80 in which a corner portion surrounded by the dashed line 82 and a corner portion surrounded by the dashed line 83 are rounded, as illustrated in FIG. 6C.
On-state current which is one of electric characteristics of a field-effect transistor depends on the width of a region where a source electrode and a drain electrode face each other. Therefore, when corner portions of ends facing each other of the source electrode and the drain electrode are rounded, the width of the region where the source electrode and the drain electrode face each other is reduced, whereby the on-state current is decreased. Further, variation in electric characteristics is caused between field-effect transistors.
In view of the above, a photolithography step using a photomask in which a corner portion of a light-blocking portion is provided with an auxiliary pattern is disclosed, by which a corner portion of a resist mask is prevented from being rounded (Patent Documents 1 and 2).